Active Pixel Sensors (APS) are solid state imagers wherein each pixel contains a photo-sensing means with associated active circuitry elements. These active circuitry elements typically are means to perform a pixel reset function, or some means to transfer charge, means to perform a voltage conversion, or circuitry elements used in amplification. APS devices have been operated in a manner where each line or row of the imager is selected and then read out using a column select signal (analogous to a word and bit line in memory devices respectively). Prior art devices have been disclosed in which all of these components have been located entirely within a single pixel boundary.
Inclusion of these active circuit element components in each pixel reduces the fill factor for the pixel because it takes up area that could otherwise be used for the photodetector. This reduces the sensitivity and saturation signal of the sensor which in turn adversely affects the photographic speed and dynamic range of the sensor, performance parameters that are critical to obtaining good image quality. Additionally, inclusion of these active circuit elements within the pixel places a limitation on the minimum size of the pixel, which adversely affects the size and cost of the image sensor.
In order to build high resolution, small pixel APS devices, it is necessary to use sub .mu.m CMOS processes in order to minimize the area of the pixel allocated to the row select transistor and other parts of the amplifier in the pixel. In essence, it takes a more technologically advanced and more costly process to realize the same resolution and sensitivity APS device when compared to a standard charge coupled device (CCD) sensor. However, APS devices have the advantages of single 5V supply operation, lower power consumption, x-y addressability, image windowing and the ability to effectively integrate signal processing electronics on-chip, when compared to CCD sensors.
A typical prior art APS pixel is shown in FIG. 1. The pixel comprises a photodetector 14, that can be constructed from either a photodiode or photogate technology, a transfer gate 15, a floating diffusion 16, reset transistor 18 with a reset gate 19, a row select transistor 8 with a row select gate 9, and signal transistor 7 which is a source follower amplifier. Inclusion of all these components within a single pixel results in a reduction in the fill factor, sensitivity and minimum size of the pixel.
Referring to FIG. 2A in conjunction with FIG. 2B, one approach to providing an image sensor with the sensitivity of a CCD and the advantages of an APS device, is to improve the fill factor and sensitivity of an APS device by reducing the amount of area allotted to components within a single pixel while maintaining the desired features and functionality of the pixel architecture.
Referring to FIG. 2A in conjunction with FIG. 2B, U.S. patent application Ser. No. 08/808,444, entitled "Active Pixel Sensor With Inter-Pixel Function Sharing" by Guidash discloses a manner in which fill factors for APS devices can be increased. This prior art device of Guidash teaches the sharing of various components typically employed within an Active Pixel Sensor. Sharing of the floating diffusion, source follow amplifier, row select transistor, and reset transistor between two row adjacent photodetectors and transfer gates are disclosed here to assist in increasing the fill factor of the pixel architecture. The basic concept utilized by Guidash for increasing fill factor is the fact that a row at a time is read out during operation of the sensor. Accordingly, Guidash was able to provide a single floating diffusion 26 and a single amplifier 27 for pixels located in two adjacent rows, instead of requiring one for every pixel as in the APS device shown in FIG. 1. Since only one row is read out at a time, a single floating diffusion 26, reset transistor 28, row select transistor 29 and signal transistor 27 (typically a source follower transistor) can be used for two adjacent pixels in separate rows.
While allowing for the sharing of components and increasing the fill factors within active pixel sensors, the device shown in FIG. 2 does not allow for the combining of function between both rows and columns, and accordingly the increase in fill factor that would result from such an architecture.
It should be readily apparent from the foregoing discussion that there remains a need within the art for an APS architecture that will allow for the combining of electrical functions between row as well as column pixels and the resulting increase in fill factor.